Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U56: SmB6 and Kondo Insulators |
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Sponsoring Units: DCMP Chair: Xiaxin Ding, Idaho National Laboratory Room: Mile High Ballroom 2C |
Thursday, March 5, 2020 2:30PM - 2:42PM |
U56.00001: Kondo holes break topological protection of heavy Dirac fermions in SmB6 Harris Pirie, Yu Liu, Eric Mascot, Pengcheng Chen, Shanta Saha, Xiangfeng Wang, Johnpierre Paglione, Mohammad H Hamidian, Cyrus F. Hirjibehedin, Dirk Klaus Morr, Jennifer E. Hoffman The interplay of magnetism and strongly correlated topology is expected to generate exotic quantum phenomena. SmB6 is a leading platform, as it was recently shown to host heavy Dirac surface states that form within a screened Kondo lattice [1]. In conventional heavy fermion materials, it is well known that local magnetic structure is nucleated by Kondo holes–substitutions on the f-electron contributing lattice. If the same mechanism exists in the topological counterparts to these systems, Kondo holes would provide a local gauge of the impact of magnetism on the emergent heavy Dirac fermions. However, current experimental techniques to image the microscopic signature of a Kondo hole are challenging, and require tracking the complex heavy fermion band structure. Here, we introduce electrochemical potential imaging spectroscopy to directly visualize the impact of Kondo holes from the liberation of screening electrons around them. In SmB6, we discover that Sm-site defects create Kondo holes that locally break topological protection, allowing backscattering of heavy Dirac fermions. |
Thursday, March 5, 2020 2:42PM - 2:54PM |
U56.00002: Surface state properties of SmB6 under pressure Soonbeom Seo, Yongkang Luo, Sean Thomas, Zachary Fisk, Joe D Thompson, Priscila Rosa, Filip Ronning Electronic transport in the Kondo insulator SmB6 is dominated at low temperatures by a surface state, which could arise as a consequence of a non-trivial topology of its bulk electronic structure. At ambient pressure, our thermopower measurements have suggested the surface states of SmB6 to have a strongly enhanced effective mass, consistent with recent quasiparticle interference studies from STM. Pressure increases the hybridization between f- and conduction electrons eventually leading to a metallic state. Here we report thermopower measurements under pressure that illustrate that the Fermi energy of the surface state is unchanged as a function of pressure in the insulating regime. We discuss our results in consideration of Hall effect data that reveal a carrier density increase with increasing pressure. |
Thursday, March 5, 2020 2:54PM - 3:06PM |
U56.00003: A Resonant Inelastic X-ray Scattering investigation of the 4f states in SmB6 Andrea Amorese, Oliver Stockert, Kurt Kummer, Nicholas B Brookes, Dae-Jong Kim, Zachary Fisk, Maurits Haverkort, Peter Thalmeier, Liu Hao Tjeng, Andrea Severing The crystal-field (CF) splitting of the 6H5/2 Hund's rule ground state of Sm3+ in the strongly correlated topological insulator SmB6 [1,2] has been determined with high resolution resonant inelastic x-ray scattering (RIXS) at the Sm M5 edge [3]. The valence selectivity of RIXS allows to isolate the crystal-field-split excited multiplets of the Sm3+(4f5) configuration from those of Sm2+(4f6) in intermediate valent SmB6. We find that the quartet G8 ground state [4] and the doublet G7 excited state are split by DCF=20±10meV. Considering this as an upper limit for the 4f bandwidth suggests an extremely large mass renormalization from the band structure value, which can be linked to the small coefficient of fractional parentage for the hopping of the 4f electrons [5]. The tiny band width complies with the small value of the gap and may be used to put constraints to the energies of the topological surface states. |
Thursday, March 5, 2020 3:06PM - 3:18PM |
U56.00004: A Parameter-free Study of the Magnetic and Electronic Structure of the Topological Heavy-fermion Compound SmB6 Ruiqi Zhang, Bahadur Singh, Christopher Lane, Jamin Kidd, Yubo Zhang, Bernardo Barbiellini, Robert Markiewicz, Arun Bansil, Jianwei Sun We revisit the highly debated mixed-valence compound SmB6, which has been widely studied in both theory and experiment. One outstanding challenge is the difficulty in obtaining accurate theoretical band structures of SmB6 as the starting point for further theoretical studies. We report new first-principles electronic-structure results, finding a band structure in agreement with experiments, wherein the correct crystal field splitting of f electrons is captured by considering magnetic configurations. Intermediate valency and the right ground state symmetry are also captured in our calculations. The energies for each magnetic ordering are extremely close, indicating that SmB6 exhibits spin fluctuations. Moreover, our results confirm that SmB6 is a robust Kondo topological insulator due to its strong d and f band hybridization. Our study paves the way for future mean-field theory investigations of electronic and atomic structures, phase diagrams, and band topology of f-electron systems and other complex materials. |
Thursday, March 5, 2020 3:18PM - 3:30PM |
U56.00005: Conductive dislocation lines in SmB6 observed through microwave impedance microscopy Kejun Xu, Jing Xia, Jan Zaanen, Zhixun Shen SmB6 is a candidate for an important class of topological insulator arising from strong correlations. Transport studies suggest surface-dominated transport at low temperatures [1]. Photoemission also revealed 2D in-gap states [2], however the topological nature of these states are under debate [3]. Here we use scanning microwave impedance microscopy to reveal conductive dislocation lines that terminate at step edges of the SmB6 surface. Temperature dependence show that these states persist up to high temperatures beyond any Kondo energy scale. These observations reveal an important conduction channel that must be taken into account for transport studies. Further studies are desired to probe the topological nature of these 1D dislocation states. |
Thursday, March 5, 2020 3:30PM - 3:42PM |
U56.00006: Electrical Transport Properties of SmB6 Nanowires Sujoy Ghosh, Sean Thomas, Yeonhoo Kim, Filip Ronning, Eric Bauer, Jinkyoung Yoo, Priscila Rosa Strongly correlated electron systems based on rare-earth elements have attracted a great deal of interest in the last few decades due to their emergent properties, such as valence fluctuations, metal–insulator transitions and heavy-fermion superconductivity near the quantum critical point. Among these, SmB6 is a topological Kondo insulator candidate, an ideal platform to investigate the interplay between topology and strong interactions as well as possible exotic quasiparticles, such as Majorana fermions. In this presentation, I will talk about our recent experimental efforts on the synthesis and characterization of SmB6 nanowires prepared via a low-temperature reaction. I will further discuss the electrical transport properties of these nanowires under different temperatures and magnetic fields. |
Thursday, March 5, 2020 3:42PM - 3:54PM |
U56.00007: Synthesizing SmB6 single crystals by various techniques Shanta Saha, Lucas William, Wesley T Fuhrman, Yun Suk Eo, Ryan Baumbach, Johnpierre Paglione Samarium Hexaboride (SmB6), a mixed valence Kondo insulator, has attracted much recent attention due to the discovery of a low temperature conducting surface state, proposed to be a topologically protected surface state that arises due to the formation of a Topological Kondo insulator state. To date, single crystals of SmB6 have only been synthesized by ‘aluminum flux’ and ‘floating zone’ methods, mainly due to the high melting point of boron. Recently, quantum oscillations were observed in the both type of crystals, but intrinsic origins of those findings are controversial due to possible extrinsic origins such as aluminum inclusions. Therefore, growing single crystals using alternative methods is considered important for elucidating the intrinsic electronic properties of SmB6. We will discuss efforts to produce SmB6 single crystals by radiofrequency (RF) technique and by flux growth methods using alternative fluxes, comparing the results of various techniques. |
Thursday, March 5, 2020 3:54PM - 4:06PM |
U56.00008: Revisiting Hall coefficient measurements in samarium hexaboride Alexa Rakoski, Yun Suk Eo, Dmitri Mihaliov, Cagliyan Kurdak, Priscila Rosa, Zachary Fisk, Boyoun Kang, Myung-suk Song, Beongki Cho, Monica Ciomaga Hatnean, Geetha Balakrishnan Samarium hexaboride is a correlated material in which strong f-d interactions open a small gap at the Fermi energy below ~100 K. In transport, SmB6 demonstrates activated behavior down to ~4 K, terminated by a conduction mechanism attributed to the emergence of a topological surface state. We present a study of transport, especially Hall coefficient, from three perspectives. First, we discuss a method of analyzing Hall data at low temperatures, allowing for accurate extraction of mobility from the conductive surface. Next, we re-examine a well-known high-temperature feature in Hall data: the sign change from negative to positive as temperature is raised above ~65 K. This sign change has been attributed to skew scattering due to the strong f-d interactions, but it leads to an inaccurate determination of carrier density and mobility. We discuss a method to improve the accuracy of transport analysis that relies on ARPES data at high temperature to correct for the positive Hall sign. Finally, we present a method of carving a micron-sized Hall bar from SmB6 to investigate the effect of sample size on transport parameters. Taken together, these results aim to provide a summary of current transport perspectives on SmB6. |
Thursday, March 5, 2020 4:06PM - 4:18PM |
U56.00009: Properties of the donor impurity band in mixed valence insulators Brian Skinner In traditional semiconductors with large effective Bohr radius, an electron donor creates a hydrogen-like bound state just below the conduction band edge. The properties of the impurity band arising from such hydrogenic impurities have been studied extensively during the last 70 years. Here I consider whether a similar bound state and a similar impurity band can exist in mixed valence insulators, such as SmB6 and YbB12. I show that the structure of the hybridized conduction band leads to an unusual bound state that can be described using the physics of the one-dimensional hydrogen atom. The properties of the resulting impurity band are consequently modified in a number of ways relative to the traditional semiconductor case; most notably, the impurity band can hold a much larger concentration without inducing an insulator-to-metal transition. I give an estimate of the DC and AC conductivities and specific heat associated with the impurity band, and show that they are consistent with experiments on SmB6. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U56.00010: Topological surface state of Kondo insulator YbB12 Yuki Sato, Shigeru Kasahara, Yuichi Kasahara, Ziji Xiang, Colin B Tinsman, Lu Chen, Lu Li, Takasada Shibauchi, Fumitoshi Iga, John Singleton, Nityan Nair, Nikola Maksimovic, James Analytis, Yuji Matsuda YbB12 is a Kondo insulator which is theoretically predicted to host metallic surface states arising from the nontrivial band structure. Recent observation of quantum oscillations (QOs) in YbB12 arouses great interest since it may indicate a Fermi surface in an insulator phase. The most important subject is to clarify whether the QOs arise from insulating bulk or metallic surface. Here we fabricated micro-devices of YbB12 using focused ion beam technique and measured transport properties in high fields. All of the samples fabricated by FIB and mechanical polishing exhibit resistivity plateaus at low temperatures and residual resistivity decreases linearly as a function of thickness. These results provide evidence of 2D surface conduction in YbB12. We also find the device shows positive and quasi-linear H-dependent magnetoresistance, which is attributed to the anti-weak localization of the surface electrons with spin-momentum locking. No discernible QOs are observed in this device, suggesting the QOs in single crystals arise from insulating 3D bulk. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U56.00011: Field-induced “exotic metal” phase in Kondo insulator YbB12 Ziji Xiang, Yuichi Kasahara, Lu Chen, Tomoya Asaba, Yuki Sato, Colin B Tinsman, Fumitoshi Iga, John Singleton, Yuji Matsuda, Lu Li YbB12, a representative Kondo insulator, has attracted attention in recent years as it exhibits Shubnikov-de Haas effect in intense magnetic fields. With the magnetic field further increasing, YbB12 goes through an insulator-to-metal transition. We systematically studied the high field metallic phase in YbB12 in pulsed magnetic fields. The temperature dependence of the resistivity in this phase reveals a crossover from non-Fermi-liquid behavior above 4 K to Fermi-liquid-like behavior below 2 K. The low-temperature state also shows unusually large Kadowaki-Woods ratio. Results of the proximity detector oscillator measurements provide evidences that the electronic structure of YbB12 in the metallic phase depends on both the strength and the orientation of external magnetic fields. |
Thursday, March 5, 2020 4:42PM - 4:54PM |
U56.00012: Magnetic Field Induced Fermi Liquid In a Candidate Topological Kondo Insulator Satya Kushwaha, Mun K. Chan, Joonbum Park, Sean Thomas, Eric Bauer, Joe D Thompson, Filip Ronning, Priscila Rosa, Neil Harrison Kondo insulators are expected to undergo an insulator-to-metal phase transition under strong magnetic fields. We show that Ce3Bi4Pd3 is a narrow gap Kondo insulator. Our high magnetic pulse-field transport and magnetization experiments reveal the destruction of Kondo coupling and the closing of the Kondo gap at a critical magnetic field, Bc ≈ 11 T. A Fermi liquid state emerges for fields beyond Bc, with a decrease in electronic correlations. |
Thursday, March 5, 2020 4:54PM - 5:06PM |
U56.00013: First-principles study of band topology for f-electron materials: Ce3(Pt/Pd)3Bi4 Jamin Kidd, Ruiqi Zhang, Jianwei Sun Recent density functional theory (DFT) calculations on the proposed topological Kondo insulator SmB6 using the newly developed SCAN functional have yielded valence f-band splitting consistent with experiment, as well as band hybridization and Z2 indices for the paramagnetic phase by following a symmetry analysis of the periodic Anderson Kondo lattice Hamiltonian.1 Motivated by these promising results, we apply the SCAN functional to study the ternary Ce3(Pt/Pd)3Bi4 systems, a new family of candidates for strongly correlated topological materials. While the Pt-based (Pd-based) compound has been predicted to be a Kondo insulator (Kondo semimetal), the topological properties of both systems are still under debate.2,3 Here, we consider the local magnetic moment of Ce atoms and calculate the electronic structures for typical magnetic configurations. We then identify the topological phases present in both materials by calculating the appropriate invariants and comparing our results to previous experiments. |
Thursday, March 5, 2020 5:06PM - 5:18PM |
U56.00014: Optical Properties of Kondo Insulators Ari Wugalter, Yashar Komijani, Piers Coleman This talk will discuss the optical properties of Kondo insulators, including the dielectric constant and magnetic permeability. Using a large-N Abrikosov fermion representation in a treatment of a Kondo lattice model, we show that the dielectric constant is determined by the squared ratio of the plasma frequency to the direct gap, predicting dielectric constants of order 1000, diffractive indices of order 30. By treating the magnetic correlations of the f-electrons as a gapless spin-liquid, described by dispersing f-electrons coupled to a gauge field we derive an effective model for the gauge fields which includes a continuum of excitations as well as a hybridization between the external electro-magnetic gauge field and the emergent gauge field. The gradient expansion of the effective model predicts a massless `light’ mode and a massive Higgs mode. The observation of this Higgs mode would provide an interesting opportunity to establish the presence of spin fractionalization and emergent gauge fields in Kondo insulators. |
Thursday, March 5, 2020 5:18PM - 5:30PM |
U56.00015: Interplay between magnetism and topology in Kondo Insulators David Riegler, Michael Klett, Seulgi Ok, Titus Neupert, Peter K Woelfle, Ronny Thomale Topological Kondo insulators are a rare example of an interaction-enabled topological phase of matter, making them an intriguing but also hard case for theoretical studies. |
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